Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock'

Rafael J. Wysocki
2015-09-25T20:30:33Z

On Friday, September 25, 2015 01:25:49 PM Viresh Kumar wrote:
> On 25 September 2015 at 13:33, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > You're going to change that into bool in the next patch, right?
>
> Yeah.
>
> > So what if bool is a byte and the field is not word-aligned
>
> Its between two 'unsigned long' variables today, and the struct isn't packed.
> So, it will be aligned, isn't it?
>
> > and changing
> > that byte requires a read-modify-write. How do we ensure that things remain
> > consistent in that case?
>
> I didn't understood why a read-modify-write is special here? That's
> what will happen
> to most of the non-word-sized fields anyway?
>
> Probably I didn't understood what you meant..

Say you have three adjacent fields in a structure, x, y, z, each one byte long.
Initially, all of them are equal to 0.

CPU A writes 1 to x and CPU B writes 2 to y at the same time.

What's the result?

Thanks,
Rafael